1. Field of the Invention
The present invention relates to a hydraulically damped mounting device. Such a mounting device usually has a pair of chambers for hydraulic fluid, connected by a suitable passageway, and damping is achieved due to the flow of fluid through that passageway.
2. Summary of the Prior Art
In EP-A-0172700, a hydraulically damped mounting device of the "bush" type was disclosed which damped vibration between two parts of a piece of machinery, e.g. a car engine and a chassis. In a bush type of hydraulically damped mounting device, the anchor for one part of the vibrating machinery is in the form of a hollow sleeve, and the other anchor part is in the form of a rod or tube extending approximately centrally and coaxially of the sleeve. Resilient walls then interconnect the central anchor part and the sleeve to act as a resilient spring for loads applied to the mounting device. In EP-A-1072700, the resilient walls also defined one of the chambers (the "working chamber") in the sleeve, which chamber was connected via the elongate passageway to a second chamber (the "compensation chamber") bounded at least in part by a bellows wall which was effectively freely deformable so that it could compensate for fluid movement through the passageway without itself resisting that fluid movement significantly.
In GB-A-2291691, the arrangement disclosed in EP-A-1072700 was modified by providing a bypass channel from the working chamber to the compensation chamber. Under normal operating conditions, that bypass channel was closed by part of the bellows wall bounding the compensation chamber. At high pressures, however, the bellows wall deformed to open the bypass channel, thereby permitting fluid from the working chamber to pass directly into the compensation chamber without having to pass through the full length of the passageway.
In both EP-A-1072700 and GB-A-2291691, the resilient walls extended generally axially along the interior of the mount. Those walls therefore formed axially elongate blocks of e.g. rubber material which were configured to achieve the desired static spring requirements. The material of the block was deformed primarily in shear, to give maximum durability. As the resilient walls also formed walls of the working chamber, the axial ends of the working chamber were closed with material being integral with the resilient walls. In practice, however, the spring effect of those ends walls was small, so that the spring characteristic of the mount could be determined by the axially extending resilient walls.